A known article for insertion into an opening is a closure plug. The plug may be moulded from plastics material and comprises a circular cap and a narrower hollow cylindrical part extending co-axially from the cap and including a resilient circumferential rib. When the plug is pushed into an opening, the rib resiliently deforms by contact with the walls of the opening and holds the plug in place. The cap covers the end of the opening.
Such plugs may only be used for one particular size of cylindrical opening, or perhaps a small range of diameters, the limits being determined by the maximum diameter of the rib and its resilience.